Engineering Geological Soil and Rock Characterization in the ...

© CNCS Mekelle University

Engineering Geological Soil and Rock Characterization in the Mekelle Town, Northern Ethiopia: Implications to Engineering Practice

Gebremedhin Berhane

Department of Earth Sciences, College of Natural and Computational Sciences, Mekelle University, P.O. Box 1202, Mekelle, Ethiopia ([email protected])

ABSTRACT The study was conducted to assess the index properties and characterize soils and rocks of Mekele town located in the northern Ethiopia having an area of 45 km2. Geological, engineering geological and geotechnical condition of the rocks and soils was studied on the bases of field description, in-situ geotechnical test and laboratory analysis. Four soil types were identified in the field: clay, silt, sandy clay/silt and clayey/silty sand soils. Laboratory result revealed that moisture content of the soils ranges from 15.8 to 40.9% for clay; 21.7 to 34.7% for silt; 6.6 to 20.5% for sandy silt/clay and 14.2 to 23% for clayey/silty sand soils. pH and electrical conductivity of the soils vary from 7.1-8.5 and 180-1930µS/cm, respectively. The liquid limit (LL) in percent ranges from 29-59, 50-67.4, 37.5-70.8 and almost non-plastic to 66 for clay, silt, sandy clay/silt and clayey/silty sand, respectively. Similarly, the plasticity index in percent (PI) of the soils ranges from 14-36.6, 13.3-37.4 and non-plastic -38.6 for clay, silt, sandy clay/silt and clayey/silty sand soils, respectively. The shrinkage limit in percent (SL) of the soils varies from 9.3 for clayey/silty sand to 30.9 for silt. The free swell of the soils is highly variable, from 0 to as high as 70%. The dolerite shows variable strength from weak rock mass strength (2Mpa) to very high rock mass strength (150Mpa); the sandstone and limestone-marl-shale intercalation show low or weak rock mass strength and the well-bedded limestone have generally high rock mass strength (60 to 160Mpa). The main geotechnical problems that affect design and related infrastructure development in the town are found to be presence of expansive soil, cyclic weak and strong rock units with depth, and variable weathering profile. Key words: Engineering geological, Liquid limit, Mekelle Town, Northern Ethiopia, Rock mass, Soils. 1. INTRODUCTION

Mekelle town is built-up over cyclic Mesozoic Sedimentary rocks and Quaternary sediments.

These materials are characterized with great diversity of genetic types and different engineering

geological behavior. Like most civil engineering structures (such as bridge, tunnels, etc) big

towns require detail knowledge of geological, geotechnical and engineering geological properties

of the foundation and the materials used for construction. Geotechnical and engineering

geological investigation and mapping mainly focus towards understanding the interrelationships

between the geological environment and the engineering situation; the nature and relationships

between the geological components, the active geodynamic processes and the prognosis of

Gebremedhin, B (MEJS) Volume 2 (2):64-86, 2010


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processes likely to result from the changes being made (UNESCO, 1976). From this perspective,

investigation of soils and rocks of a town, as a material that is used to build with or on and as a

material of the environment that may act in combination with other forces of nature (geodynamic

processes) or of civilization to affect landforms, structure, and the state of our environment is

extremely important. Collapsible and alluvial soils are found causing basic difficulties in

building and construction foundations (Vanushka Petrova and Jordan Evlogiev, 2003). The

cyclic sedimentary rocks of the Mesozoic rocks in the region are also found to be pervious and

weak in strength due to various types of discontinuities (Gebremedhin Berhane, 2010).

Mekelle was founded more than a century ago. The town is presently rapidly expanding.

Currently, many civil engineering structures such as multistory buildings, roads, bridges, etc are

under construction in the town. However as in many towns of Ethiopia, very little is known

about the soil and rock conditions or engineering geology of the town. The only research to be

mentioned is the work of Gebremedhin Berhane (2002), in which he has classified the rocks and

soils of the area based on their engineering properties in addition to geomorphological,

geological, and engineering geological and geotechnical mapping. This work was used as

baseline information for the present research.

In the light of the above point, the present research was aimed at assessing and evaluating the

engineering geological and geotechnical conditions of the town and to provide important

engineering geological data that may help to plan, design and maintain engineering projects.

Specifically, the objectives were to i) asses and analyze important engineering geological and

geotechnical properties of the soils and rocks of the town; ii) classify the soils and rocks of the

town into different engineering geological properties or units; and iii) produce geological and

engineering geological maps of the town.

2. DESCRIPTION OF THE STUDY AREA

2.1. Location

Mekelle is the capital city and commercial center of the Tigray National Regional State in the

northern Ethiopia (Fig.1). The town is located at 39033'E longitude and 13032'N latitude, situated

in the extension of the central highlands of Ethiopia. The altitude of Mekele is between 1965 m

and 2220m above sea level. The town is bounded by mountain ranges in the east and north.



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2.2. Climate

Climatically, the area is classified as "Woina Dega" (temperate) with an effective temperature

between 140C and 200C (Ethiopian Mapping Agency, EMA, 1981), which for most of the time is

comfortable. It has a moisture index (P/ET) ranging in between 0.25 and 0.5, which indicates

moderately dry area. Mekelle lies between 600mm to 700mm rainfall region. The mean annual

temperature ranges between 160C and 200C (Gebremedhin Berhane, 2002).

2.3. Topography

The altitude varies from 2220 m at eastern side to 1965 m in the northwestern side of the town

(lower reach of Illala River). The town has an overall tilt from eastern to western and

northwestern side. Most streams and tributaries are controlled by this tilt while others are

controlled by geological structures and underlying geology.

2.4. Geology

The first recorded geological work in the northern provinces of Ethiopia was done by Blanford

(1870) cited in Beyth (1971), who divided the Trap Volcanics of the Ethiopian highlands into

two units, a lower entirely basaltic Ashangi Series, and an upper Magdala Series which contained

many intercalation’s of trachyte. Then Dainelli and Marinelli (1912) and Merla and Minucci

(1943) as cited in Beyth (1971) proposed the transgression - regression phenomena to explain the

sedimentary history of the whole of the Horn of Africa, including Ethiopia. In 1970, Levitte

studied the geology of Mekele (Central part of Sheet ND37- 11) and he divided the rocks in the

area into four major units: Basement complex, Paleozoic - Mesozoic Sedimentary sequence,

Cenozoic Trap Volcanics and Sediments of the Ethiopian Rift.

Beyth (1972) done detail mapping of Northern Ethiopian provinces (Central and Western

Tigray). According to this work the history of the sedimentary basin in Tigray (Mekelle Outlier)

began in either the Ordovician or Carboniferous and probably ended in lower Cretaceous before

the eruption of the Trap Volcanics.

The main lithologic units in the Mekelle town are Quaternary sediments, dolerite, limestone-

marl-shale intercalation, sandstone and bedded limestone (Fig. 2). Description of each of the

rock units is presented below.



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2.4.1. Quaternary Sediments

These sediments consist of alluvial, colluvial deposits and residual soils. The alluvial deposit

ranges in grain-size from clay to sand with minor boulders. It is widely observed along streams

and northern and northwest of the town. It is dark to gray in color, loose to stiff and in places

stratified. Colluvial deposits are common along foot of steep slopes (east of the center of the

town). The residual soils range in grain-size from clay to sand with some inclusions of angular

boulders, mainly yellowish in color, are found in areas of gentle slopes.

Figure 1. Location map of Mekelle town (including 1:10,000-scale aerial photographs showing

two selected points within the study area).

Main Road

Palace



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Figure 2. Geological Map of Mekelle Town (including location of test pits and major road net

works, modified after Gebremedhin Berhane, 2002). 2.4.2. Dolerite

It outcrops on the eastern side of the town (forming steep cliff) and near the center of the town

(Fig. 2). It is black, fine to medium grained and is characterized by spheriodial weathering. This

rock unit is jointed (vertically and horizontally). It shows differential weathering from place to

place and in many localities remnant corestones are common especially in foundation

excavations.



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2.4.3. Limestone-Marl-Shale Intercalation

This intercalation unit covers large part of the area forming gentle slopes. It is variegated,

stratified, fine grained, friable and laminated. The limestone layer is black and in places light

yellow in color; it is stronger than the other layers. The shale and marl layers are generally gray

and light yellowish in color. In places swarms of dolerite dykes and sills are observed.

Weathering is more intensive in shale beds than limestone beds.

2.4.4. Sandstone

It is white to light yellowish in color, friable, less cemented, bedded and weathered. On hand

specimen quartz grains are dominant, with some dark minerals. It is fine to medium grained, in

places interbeds of siltstone are observed, shows weak to fairly strong effervescence upon a drop

of 10% hydrochloric acid solution indicating the presence of calcite as cementing material.

2.4.5. Bedded Limestone

It is well bedded, black and yellowish in color, crystalline and slightly weathered. Traces of

fossils and shell fragments are observed in some hand specimens. Bed thickness is variable (1m

to 3m). Thin layers of shale and marl are found in this unit, i.e. alternating limestone and thin

beds of shale and marl.

2.5. Geological Structures

The dominant structures in the Mekelle town are faults, joints and bedding planes. The faults are

interpreted from air photographs (verified in the field), while the other structures are observed

and some measurements were taken in the field. The study revealed WNW-ESE, N-S and NNE-

SSW with minor E-W striking faults (Fig. 3). Joints are other structures common in the study

area. The strikes of the joints are generally parallel to the faults of the area and seldom

perpendicular. Most of the joints are vertical and some are horizontal, parallel to bedding planes

in sedimentary rocks. Figure 3 shows rose diagram of vertical joints measured in the study area.

Two major joint sets are observed (NNE & WNW trending) with some minor sets (NE & NW

strike, Fig. 3). The third geologic structure is bedding plane, which is considered as discontinuity

in engineering geological investigations and could facilitate landslide or slope instability. In



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large part of the town beds are horizontal, but inclined beds are also observed resulted from

dolerite intrusion and faulting.

Figure 3. Rose diagram of joints (Gebremedhin Berhane, 2002).

3. METHODOLOGY

The research work involved a number of fieldworks in different seasons (2001 - 2005 and 2008).

During these fieldworks rock and disturbed soil samples were collected and analyzed for various

index and engineering parameters. Existing geotechnical and engineering geological data were

collected from different organizations and individuals and preliminary photo-geological map

were produced by photo interpretation using stereo-pairs of panchromatic black and white aerial

photographs of scale 1: 10,000 which were flown in 1994.

Continuous rock and soil descriptions, test pit excavation (up to a depth of 6m) for in-situ

observations and sample collection, and discontinuity measurements were conducted. Pocket

penetrometer test and Schmidt hammer rebound test on soil and rock respectively were carried

out. Samples were analyzed in Mekelle geotechnical laboratory according to procedures and

methods proposed by American State Testing Materials (ASTM). The rocks and soils of the

town were classified on the basis of their engineering behavior according to Unified Soil

Classification Systems (USCS) and classification proposed by International Association of

Engineering Geologists (IAEG, 1981).

N,360

Strike Direction: 15 ° classes

90270

180

Num total: 114



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4. RESULTS AND DISCUSSION

4.2. Description, index properties and classification of soils

4.2.1. Lithological / Textural Description of the Soils

Even though portion of the existing buildings in Mekelle town are dominantly founded on the

limestone-marl-shale intercalation, limestone and dolerite units, large part of the town is covered

by soil of up to more than 10 m thick. The soils are lithologically grouped into clay, silt, sandy

clay/silt and clayey/silty sand soils.

Clay soils are generally dark / black in color, mainly observed in the northern part of the town

and partly in the central part. The silt soils are variable in color (dark, gray and yellowish), found

in the northern and northwestern part of the town.

The sandy clay/silt soils are mainly residual in origin and found in the north and to southern part

of the town. Its color is yellowish to dark-gray. The clayey/silty sand soils have very limited area

coverage, as pockets with light gray to dark color.

4.2.2. Natural Moisture Content (NMC)

For coarse and fine-grained soils, water content can have a significant effect on the soils

behavioral properties when used for construction purposes and foundations. Moisture content

affects the settlement (consolidation) condition; shear strength and suitability of soil for

compaction. Moreover, the swelling-shrinkage condition of a particular soil is related to its

moisture content and its change with time. Consistency of a fine grained soil also depends

largely on its moisture content. Samples were collected and immediately submitted to determine

natural moisture content (NMC) of soils. The result (Table 1) shows that the clay soils have a

moisture content of 15.8 – 40.9%, silt soils have 21.7 – 34.7%, sandy clay / silt have 6.6 – 20.5%

and the clayey / silty sand soils have 14.2 –23%.



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Table 1. Natural Moisture content, pH, electrical conductivity and percent dispersion of soils of Mekelle town.

4.2.3. Electrical Conductivity (EC) and pH of Soils

Chemical tests are normally preformed on soils to ascertain whether the soil is acidic, alkaline or

neutral (Abramson et al, 1996). pH and electrical conductivity of soils of Mekelle town were

analyzed in the laboratory from extracts (Table 1). The electrical conductivity (opposite to

specific electrical resistance) of the soil is one of the most important factors in determining soil

aggressiveness. As the electrical conductivity of a soil increases its aggressiveness increases

(Abramson et al, 1996). Soils pH values vary from 7.1 (about neutral) to 8.5 (alkaline). This

shows that further chemical tests will be important in design and planning of engineering

structures in the alkaline soil. The EC value of the soils was found to vary from 190 to 1930 µS

/cm. The value is generally high signifying its corrosive nature.

4.2.4. Dispersion of Soils

Dispersive clays are those, which deflocculates or disintegrate when exposed to water.

Dispersive clay soils are identified by various methods. In this work the double hydrometer

method was adopted. It is similar to the normal hydrometer test, except that neither mechanical

agitation nor chemical dispersing agent is applied in this method. The percent dispersion of soils

Pit Depth (m)

Lithologic type

Moisture content (%)

pH EC (µS/ cm) % dispersion

TP1 1.5 Clay soils

24.3 8.5 250 TP4 1.0 24.9 8.2 180 TP8 2.0 40.9 7.4 900 TP9 3.0 15.8 0.016 TP11 1.5 18.3 0.013 TP3 0.5

Silt soils

34.7 7.1 1930 TP3 3.0 27.9 7.3 1840 TP6 2.4 21.7 7.6 530 TP2 0.5

Sandy clay/silt soils

20.5 7.7 190 TP10 1.5 9.95 TP12 1.5 6.6 14.33 TP12 4.0 10.4 TP13 2.0 10.6 55.03 TP14 2.0 14.7 TP5 2.0

Clayey /silty sand soils

15.3 7.8 250 TP6 1.4 23 7.5 900 TP13 1.2 14.2



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is the ratio of percent passing 5µm size without applying dispersing agents and percent passing

5µm by applying dispersing agents.

When the percent dispersion is nearly 100% it indicates a completely dispersive soil. In general

dispersive clays are highly erosive, have high shrink-swell potential and have low permeability

in an intact state. Dispersive soils are troublesome in terms of slope stability (in natural slopes

and embankments); the underlying soil mass of the slope often suffers from internal erosion

when subject to localized seepage zones. From the results (Table 1) of the test the soils seems

non-dispersive.

4.2.5. Grain-size Analysis

Two methods were used to find the particle-size distribution of the soil samples: sieve analysis,

for particle sizes larger than 0.075 mm (No. 200) in diameter; and hydrometer analysis for

particle-sizes smaller than 0.075 mm in diameter. During hydrometer analysis sodium

hexametaphosphate (NaPO3) or Calgon was used as a dispersion agent, and all analysis was

determined based on ASTM (D 421 & 422) procedure. The results of grain-size analysis are

presented in table 2. The diameter or size range is adopted from ASTM as follows: >4.75 mm

(No.4) gravel; 4.75 – 0.075 mm (No.200) sand; 0.075 – 0.002 mm silt and < 0.002 mm clay.

Table 2. Specific gravity and grain-size analysis results of the soils of Mekelle town area.

Pit Depth (m)

Lithologic type Specific gravity

Gravel Sand Silt Clay Origin

(%) TP1 0.2 -1.5

Clay soils

2.70 0 13 58 29 Residual TP4 0.15 -1.0 2.84 0 14 66 20 Residual TP8 0.3 -2.0 2.45 0 13 60 27 Alluvial TP9 0.3 -3.0 2.70 0 10 46 44 Alluvial TP11 0.2 -1.5 2.75 0 14 36 50 Residual TP3 0.2 -0.5

Silt soils 2.51 0 11 45 44 Alluvial

TP3 1.0 -3.0 2.51 0 12 56 22 Alluvial TP6 1.4 -2.4 2.60 0 25 33 42 Residual TP2 0.1 -0.5

Sandy clay/silt soils

2.71 0 28 40 32 Residual TP10 1.0 -1.5 2.70 0 38 16 30 Alluvial TP12 0.2 -1.5 2.70 0 34 40 26 Alluvial TP12 1.5 -4.0 2.75 0 18 39 43 Alluvial TP13 1.2 -2.0 2.70 0 34 38 28 Alluvial TP5 1.0 -2.0

Clayey/silty sand soils 2.90 2 73 16 8 Alluvial

TP6 0.2 -1.4 2.51 0 51 13 36 Residual TP13 0.2 -1.2 2.65 3 62 23 12 Alluvial



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4.2.6. Atterberg (consistency) Limits and Plasticity Index

The consistency of a fine-grained soil is the physical state in which it exists; it is related to a

larger extent to water content. Consistency denotes degree of firmness of the soil that is indicated

by tests in the field as soft, firm, stiff or hard. Even though it is not possible to interpret the

Atterberg limits and plasticity characteristics in fundamental terms, these parameters are of great

practical use as index properties of cohesive soils. The engineering properties (uses) of fine-

grained soils are, generally, related to these index properties. The more plastic a soil means the

more compressible, higher shrinkage-swell potential and the lower is its permeability will be

(Abramson et al, 1996).

The Atterberg limit of selected soil samples was determined in the laboratory (Table 3). From the

results of plastic (PL) and liquid limit (LL) plasticity index (PI) of the soils was calculated.

Plastic index is important in classifying fine-grained soils and is fundamental to the Casagrande

plasticity chart. The larger the plasticity index, the greater will be the engineering problems

associated with using the soil as an engineering material, such as foundation support for

residential building and road sub grades (Bowles, 1992).

The clay soils have LL: 29-59%, silt soils 50-67.4%, sandy clay / silt soils 37.5-70.7% and fine

fraction of clayey/ silty sand soils non-plastic to 66%. The PL of these soils ranges 15-25.9%,

30.2-37.9%, 22.3-33.4% and non- plastic to 27.4% respectively (Table 3). Most of the soils of

the study area fall in intermediate to high plasticity type, except some of the clayey / silty sand

soils which fall in soils of low plasticity type. Comparison of the LL and PL with the NMC

(Table 1) shows that most of the clays were in plastic phase, while others were in semisolid or

solid state (their NMC were below their plastic limit at the time of sampling). The PI of clay

soils ranges from 14 to 36.5 %; silt soils 12.1 to 32.6%; sandy clay / silt soils 13.3 to 37.4% and

clayey/silty sand soils non-plastic to 38.6%, respectively.

According to description of plasticity of fine soils in terms of range of plasticity index given by

IAEG (1983), the clay, sandy clay/silty and clayey/silty sand soils are moderately to extremely

plastic and the silty soils are moderately to highly plastic type. In general, most of the plasticity

values of the soils of Mekelle town fall in the highly plastic range (17-35%).



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Table 3. Consistency limits, plasticity index values and activity of the soils of Mekelle town.

Figure 4. Plot of soils of Mekelle town on activity chart.

Pit Depth (m)

Lithologic type LL PL PI Activity

(%) TP1 02 -1.5

Clay soils

29 15.05 13.95 0.481 TP4 0.15 -1.0 40 21.5 18.5 0.925 TP8 0.3 -2.0 54.5 25.85 28.65 1.061 TP9 0.3 -3.0 59 22.85 36.51 0.83 TP11 0.2 -1.5 40.8 22.3 25.70 0.514 TP3 0.2 -0.5

Silt soils

67.4 34.85 32.55 0.74 TP3 1.0 -3.0 50 37.94 12.06 0.548 TP6 1.4 -2.4 52 30.18 21.82 0.52 TP2 0.1 -0.5

Sandy clay/ silt soils

55.5 23.9 31.60 0.988 TP10 1.0 -1.5 37.8 24.25 13.55 0.452 TP12 0.2 -1.5 45.41 22.85 22.56 0.868 TP12 1.5 -4.0 48 22.3 25.70 0.598 TP13 1.2 -2.0 70.75 33.4 37.35 1.334 TP5 1.0 -2.0

Clayey /silty sand soils 21 16 5

TP6 0.2 -1.4 66 27.36 38.64 1.073 TP13 0.2 -1.2 28.5 20.23 8.27 0.689 TP14 0.5 – 2.0 34.37 18.52 15.85

A C T I V E S O I L S

N O R M A L S O I L S

I N A C T I V E S O I L S



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Table 4. Summary of shrinkage limit results and calculated shrinkage indexes and ratio.

Pit Depth (m) Lithologic type

Shrinkage limit

Shrinkage Index

Shrinkage ratio

Liquidity index (LI)

Consistency Index (CI)

(%) TP1 02 -1.5

Clay soils

10 5.05 0.6631 0.3369 TP4 0.15 -1.0 19.23 2.27 1.63 0.1838 0.8162 TP8 0.3 -2.0 8.69 17.16 1.92 0.5253 0.475 TP9 0.3 -3.0 20.41 2.44 1.75 -0.195 1.195 TP11 0.2 -1.5 15 7.3 1.45 -0.216 1.216 TP3 0.2 -0.5

Silt soils

12.76 22.09 2.04 -0.005 1.005 TP3 1.0 -3.0 30.95 6.99 1.35 -0.833 1.833 TP6 1.4 -2.4 22.78 7.4 1.25 -0.388 1.388 TP2 0.1 -0.5 Sandy clay/

silt soils 15.55 8.35 1.88 -0.1076 1.1076

TP10 1.0 -1.5 -1.0554 2.0554 TP12 0.2 -1.5 11.11 11.74 1.93 -0.7203 1.7203 TP12 1.5 -4.0 0.016 22.28 2 -0.4630 1.4630 TP13 1.2 -2.0 20.33 13.07 1.84 -0.61 1.61 TP5 1.0 -2.0


1.76 TP6 0.2 -1.4 9.3 18.06 1.87 -0.113 1.113 TP13 0.2 -1.2 13.33 6.9 1.97 -0.729 1.729 TP14 0.5 - 2.0 7.26 11.26 1.96 -0.241 1.241

4.2.7. Activity of the soils

The activity of the soils of Mekelle town was determined from consistency limit tests and grain-

size analysis (Table 3). In geotechnical work the term activity indicates the percentage of clay in

the fraction of soil used for Atterberg limits and the potential swell and shrinkage (volume

change) of a soil, with larger values indicating an increasing potential. Skempton (1953) cited in

Bell (1983) suggested three classes of activity (active, normal and inactive, Fig. 4). Kaolinitic

and illitic clays are usually inactive whilst montmorillonitic clays range from inactive to active.

In terms of potential expansiveness soils with activity less than 0.75 are low, 0.75-1.25 medium

and those with greater than 1.25 are highly expansive (Fig.4).

4.2.8. Shrinkage Limit (SL)

The shrinkage limit of soils of the study area was determined in laboratory (ASTM D-427). The

shrinkage parameters frequently used in soil engineering, shrinkage index and shrinkage ratio,

were also calculated from the laboratory results. Liquidity index (LI) of the soils (the nearness of

its water content to its LL) and consistency index (CI) (firmness of a soil), were also calculated

from results of LL, PL and NMC (Table 4).



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Table 5. Free swell values of the soils of Mekelle town.

Pit Depth (m) Lithologic type Free swell (%) TP1 02 -1.5

Clay soils

20 TP4 0.15 -1.0 30 TP7 0.2 -2.0 40 TP8 0.3 -2.0 70 TP9 0.3 -3.0 70 TP11 0.2 -1.5 40 AF5 1.4 60 AF23 1.5

Silt soils 60

AF29 1.3 60 TP3 0.2 -0.5

Sandy clay/ silt soils 20

TP3 1.0 -3.0 50 TP6 1.4 -2.4 50 AF5 2.50 40 AF13 1.0 70 TP2 0.1 -0.5


20 TP10 1.0 -1.5 55 TP12 0.2 -1.5 40 TP12 1.5 -4.0 50 TP13 1.2 -2.0 25 AF24 2.5 40 AF29 2.5 30 TP5 1.0 -2.0 Clayey/ silty sand 0 TP6 0.2 -1.4 65 TP13 0.2 -1.2 20 TP14 0.5 - 2.0 20

4.2.9. Free Swell

Free swell tests consists of placing a known volume of dry soil in water and noting the swelled

volume after the material settles, without any surcharge, to the bottom of a graduated cylinder.

The difference between the final and initial volume, expressed as a percentage of initial volume,

is the free swell value (Chen, 1975). Results of free swell tests of the soils of the study area are

presented in Table 5.

According to Holtz (1956) cited in Bell (1983), soils having free swell value as high as 100% can

cause considerable damage to lightly loaded structures, and soils having free swell value below

50% seldom exhibit appreciable volume change even under very light loadings. The free swell

values of the soils of the study area vary from 0 to 70% (Table 5). There are many samples with

free swell of greater or equal to 50%. Hence, considerable attention should be given in

foundation design even for light structures on such soils (TP-3, 6, 9, 10, 11, 13 and 12) because

their value shows expansiveness property.



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4.2.10. Classification of soils

Approximate assessment of the engineering properties of soils can be obtained from the index

properties after appropriate classification is made. From geotechnical or engineering geological

point of view, the classification of soil may be done with the objective of finding the suitability

of the soil for construction of structures or foundations. Such a classification should provide

some guide to the engineering performance of the soil type and should provide a means by which

soils can be identified quickly (ISRM, 1981).

In this research the Unified Soil Classification System (USCS) and classification system

proposed by IAEG (1981) which is a modified form of the Unified Soil Classification (USC) and

the British Soil Classification for engineering purposes (BSCS) were employed. The USCS is

based on both grain size (after excluding boulders and cobbles) and plasticity properties of the

soil and is applicable to many uses. The soils are broadly classified into two categories, coarse-

grained, if more than 50% of the soil is retained on No. 200 (0.075mm) sieve and fine-grained, if

more than 50 % passes No. 200 sieve. The coarse-grained soils are further subdivided based on

PI and LL (Fig. 5 and Table 6).

The classification system proposed by IAEG (1981) is also based on grading and plasticity of

soils. Grading and plasticity are divided into a number of clearly defined ranges, each of which

may be referred to by a descriptive name and letter (Fig. 6 and Table 7).

4.3. Description and Classification of Rocks

Description is the initial step in an engineering geological investigation of rock masses. In view

of this, description of rocks of Mekelle town area was carried out according to descriptive

schemes proposed by IAEG (1981). In describing rocks of the town, logs and some in-situ tests

conducted on more than 40 site investigation boreholes and Schmidt hammer rebound test on 20

test points were evaluated in addition to visual description and the use of geological hammer and

weathering grade to estimate the engineering properties of the rock masses.

4.3.1. Dolerite

Outcrop of dolerite in the town is mainly dominant in the central and eastern part. This rock unit

has variable properties that resulted from degree of discontinuity and weathering. This rock unit

is classified into highly weathered weak dolerite (low mass strength), moderately weathered



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strong dolerite (medium mass strength) and fresh to slightly weathered strong dolerite (high mass

strength).

Figure 5. Plot of soils of Mekelle town on Casagrade plasticity chart.

Table 6. Classification of soils of Mekelle town area based on USCS.

Pit/ BH

Depth (m)

Lithologic type

G* Sand Silt Clay LL PI USCS classification (%) (%) S** Soil name

TP1 02 -1.5 Clay soils

0 13 58 29 29 14 CL Lean clay TP4 0.15 –1.0 0 14 66 20 40 19 CL Lean clay TP8 0.3 -2.0 0 13 60 27 55 29 CH Fat clay TP9 0.3 -3.0 0 10 46 44 59 37 CH Fat clay TP11 0.2 -1.5 0 14 36 50 41 26 CL Lean clay AF5 1.4 0 8 42 50 51 26 CH Fat clay MS 3.8 0 6 39 55 81 40 CH Fat clay MS2 1.9 0 12 46 42 59 31 CH Fat clay TP3 0.2 -0.5

Silt soils

0 11 45 44 67 33 MH Elastic silt TP3 1.0 -3.0 0 12 56 22 50 12 MH Elastic silt TP6 1.4 -2.4 0 25 33 42 52 22 MH Elastic silt with sand MS4 5.6 0 8 44 48 71 35 MH Elastic silt MS2 5.3 0 9 52 39 67 31 MH Elastic silt TP2 0.1 -0.5


0 28 40 32 56 32 CH Clay with sand TP10 1.0 -1.5 0 38 16 30 38 14 CL Sandy lean clay TP12 0.2 -1.5 0 34 40 26 45 23 CL Sandy lean clay TP12 1.5 -4.0 0 18 39 43 48 26 CL Lean clay with sand TP13 1.2 -2.0 0 34 38 28 71 37 CH Sandy fat clay AF5 2.5 0 21 47 32 43 18 CL Lean clay with sand AF13 2.5 0 28 52 18 41 18 CL Lean clay with sand AF24 2.5 0 30 48 22 40 13 ML Sandy silt TP5 1.0 -2.0 Clayey /

silty sand soils

2 73 16 8 21 5 SC-SM

Silty, clayey sand

TP6 0.2 -1.4 0 51 13 36 66 39 SC Clayey sand TP13 0.2 -1.2 3 62 23 12 29 8 SC Clayey sand AF29 2.5 0 67 24 9 36 15 SC Clayey sand

* Gravel, ** symbol.



80

L O W ( L )

C L - M L

C L

C I

C H

C VC E

M L M IM H

M V

M E

U ( U P P E R P L A S T IC IT Y R A N G E )

IN T E R M E -D IA T E

( I ) H IG H V E R Y H IG H

(V )

E X T R E M E L Y H IG HP L A S T IC IT Y

(E )

A - L IN E

PLA

STIC

ITY

IND

EX (%

)

L IQ U ID L IM IT ( % )

0

1 0

2 0 3 0 4 0

5 0

6 0

7 0

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0

C = C L A Y M = S IL T L = L O W H = H IG H I = IN T E R M E D IA T EV = V E R Y H IG H E = E X T R E M E L Y H IG H

(H )

M E K E L E T O W N S O IL S

Figure 6. Plots of soils of Mekelle town on plasticity chart based on IAEG.

Table 7. Classification of soils of Mekelle town area based on IAEG. Pit/ BH

Depth (m)

Lithologic type

G* Sand Silt Clay LL PI IAEG (1981) (%) (%) S** Soil name

TP1 0.2 -1.5 Clay soils

0 13 58 29 29 14 CL CLAY of low plasticity TP4 0.15-1.0 0 14 66 20 40 19 CI CLAY, of intermediate plasticity TP8 0.3 -2.0 0 13 60 27 55 29 CH CLAY of high plasticity TP9 0.3 -3.0 0 10 46 44 59 37 CH CLAY, of high plasticity TP11 0.2 -1.5 0 14 36 50 41 26 CI CLAY of intermediate plasticity AF5 1.4 0 8 42 50 51 26 CH CLAY of high plasticity MS 3.8 0 6 39 55 81 40 CV CLAY of very high plasticity MS2 1.9 0 12 46 42 59 31 CH CLAY of high plasticity TP3 0.2 -0.5

Silt soils

0 11 45 44 67 33 MH SILT of high plasticity TP3 1.0 -3.0 0 12 56 22 50 12 MH SILT of high plasticity TP6 1.4 -2.4 0 25 33 42 52 22 MH SILT of high plasticity MS4 5.6 0 8 44 48 71 35 MV SILT of very high plasticity MS2 5.3 0 9 52 39 67 31 MH SILT of high plasticity TP2 0.1 -0.5


0 28 40 32 56 32 CH CLAY of high plasticity TP10 1.0 -1.5 0 38 16 30 38 14 CS

(CLS) CLAY, sandy, of low plasticity

TP12 0.2 -1.5 0 34 40 26 45 23 CS (CLS)

CLAY, sandy, of low plasticity

TP12 1.5 -4.0 0 18 39 43 48 26 C (CI) CLAY, sandy, of intermediate plasticity

TP13 1.2 -2.0 0 34 38 28 71 37 CS (CLS)

CLAY, sandy, of very high plasticity

AF5 2.5 0 21 47 32 43 18 C (CI) CLAY of intermediate plasticity AF13 2.5 0 28 52 18 41 18 C (CI) CLAY of intermediate plasticity AF24 2.5 0 30 48 22 40 13 M (ML) SILT of low plasticity TP5 1.0 -2.0 Clayey /

silty sand soils

2 73 16 8 21 5 SM (SML)

SAND, silty, of low plasticity

TP6 0.2 –1.4 0 51 13 36 66 39 SC (SCH)

SAND, clay, of high plasticity

TP13 0.2 -1.2 3 62 23 12 29 8 SC (SCL)

SAND, clayey, of low plasticity

AF29 2.5 0 67 24 9 36 15 CS (CLS)

CLAY, sandy, of intermediate plasticity



81

4.3.1.1. Highly weathered, Weak Dolerite (low mass strength)

This rock is found along the eastern side of the town and the central part of the town. It is dark

greenish, medium to coarse grained, and moderately to highly weathered. The material strength

was estimated in the field and varies from 2 to 20 MPa. Joints of WNW (2750, 2850 and 2900)

strike are common, in places NNE strike joints are observed. The joints are in general vertical

and the spacing varies from 4 to 20cm. Joint surfaces are planar and rough, and very narrow to

moderately narrow aperture. Clay, calcite and silica are common infill materials. Joints were dry

during the time of investigation (February 2002 and January 2004).

4.3.1.2. Moderately Weathered, Medium Strong Dolerite (medium mass strength)

This rock is slightly stronger than the above subunit, mainly found along the eastern part and

along the central part of the town (Fig. 7). It is dark-greenish in color; medium to coarse grained

and moderately weathered. The strength of the rock varies between 13 to 45 MPa. Two sets

(WNW and NNE and random joints are dominant which are vertical and medium to widely

spaced. The joint surfaces for the systematic once, is planar and rough. The separation or

aperture is variable and ranges from very narrow to moderately narrow. Clay and calcite are the

common infill materials, occasionally silica veins are observed.

4.3.1.3. Fresh to slightly weathered, Strong Dolerite (high mass strength)

This engineering geological subunit forms steep cliff in the eastern side of the town. The rock

material strength varies from 45 to 150 MPa. Two sets of joints are dominant; WNW and NNE.

In places columnar joints are observed. The joint wall or surface is planar and smooth, and the

aperture varies from tight to narrow.

4.3.2. Sandstone (moderately weathered and weak)

It covers limited area in the southern side of the town. It is friable and less cemented. It is whitish

to light yellowish in color, medium to coarse grained and slightly to moderately weathered. Its

rock material strength is weak, estimated about 105 MPa. Three sets of joints (2850, 0200 and

0850 strike) are dominant. The WNW strike joints are closely spaced while the other two sets are

widely spaced, but all the joints are vertical. The joint wall or surface is planar and rough and the



82

aperture varies from tight to moderately wide (0 to 2.5 cm). Calcite and silica are observed as

infill material. It is horizontally bedded with bed thickness up to 0.5m.

4.3.3. Limestone-Marl-Shale Intercalation (moderate-highly weathered and weak)

Though the strength of each layer varies from weak (shale) to strong (limestone), it is considered

as weak engineering geological unit or rock with low mass strength. It is variegated (gray,

yellowish, dark brown, etc), crystalline and slightly to highly weathered. The intact rock strength

is variable, from very weak for shale to strong for limestone. Three vertical joint sets (3200, 0450

and 3600) and horizontal joint are observed in this intercalation unit. The joint surfaces or wall is

generally planar and rough, and tight to moderately narrow aperture (0-3 cm). The spacing of the

systematic joint set is on average 0.5 to 1.5 m. This intercalation unit is horizontally to sub-

horizontally bedded with bed thickness of 0.2 to 1.5 m. In places the beds are inclined in

different direction due to the intrusion of dolerite.

4.3.4. Limestone (slightly weathered, strong with high mass strength)

This rock covers limited area, mainly outcrop along streams. It is black and in places light

yellowish in color, finely crystalline and fresh to slightly weathered. The rock material strength

ranges from 62 to 160 MPa. Three vertical joint sets (3200, 0450 and 3600) and horizontal joint

parallel to bedding planes are the dominant. The joint surface or wall is planar and rough to

smooth, and the aperture varies from tight (for horizontal joints) to 5 cm (for vertical joints). The

vertical joints are widely spaced (0.5 to 2.5 m). This limestone unit is bedded; in most places

horizontal and bed thickness reaches up to 1 m.

In addition to the above-mentioned joints or discontinuities in the most engineering geological

units, large fractures or faults and intrusions of dolerite also affects the rock mass strength (Figs.

3 and 7) and the presence of most of joints are reflections of these large structures. The strike of

most of the joints is almost the same to the strike of large structures or faults. Based on the

description of the soils and rocks from field and laboratory data, delineation of engineering

geological units was made to produce an engineering geological map for the town with

appropriate legend which signifies the engineering behavior of each unit (Fig. 7).



83

Figure 7. Engineering geological map of Mekelle town.



84

5. CONCLUSIONS AND RECOMMENDATIONS

5.1. Conclusions

Geological and Engineering geological map of the town was produced at a scale of 1:10,000.

Four lithological /soil types were identified: clay, silt, sandy clay/silt and clayey/silty sand soils.

The moisture content varies from 15.8-40.9% for clay; 21.7-34.7% for silt; 6.6-20.5% for sandy

silt/clay and 14.2-23% for clayey/silty sand soils. Grain size analyses of the soils indicate that

most of the soils of the town are composed of fine fractions.

Liquid limit of the soils varies from 29-59% for clay; 50-67.4% for silt; 37.5-70.8% for sandy

clay/silt and non-plastic to 66% for clayey/silty sand soils. Similarly the plasticity index of the

soils varies from 14-36.6% for clay; 12-32.6% for silt; 13.3-37.4% for sandy clay/silt and non-

plastic to 38.6% for clayey/silty sand soils. Hence, the soils are generally moderately plastic to

extremely plastic. Based on liquid limit the plasticity of the soils varies from low to extremely

high. The consistency index of most soils is above unity and their liquidity index is below zero,

indicating the swelling behavior of most of the soils in the area. The free swell of the soils of the

area is highly variable from 0 to as high as 70%, indicating the potentially expansive behavior

and needs considerable attention in design of even light engineering structures.

Classification of soils and rocks of the town was also carried out. Based on USC system the clay

soils fall in CL (inorganic clays of low to medium plasticity, silty clays and lean clays) and CH

(fat clays) type; silt soils in MH (inorganic silts, elastic silts); sandy clay/silt soils fall in CH, CL

and ML (inorganic silts, very fine sand, rock flour, silty or clayey fine sands) and the clayey/silty

sand soils fall in SC (clayey sand, sandy clay mixtures) and SC-SM (silty clayey sand soils) type.

The lithological field description and classifications made are generally in agreement with

laboratory results. The potential expansion behavior or swelling nature of the soils is found to be

low to very high and about 50% of the soil samples show medium to high degree of expansion.

Classification of rocks was made by considering lithologic units as main groups and by

considering factors or parameters, which affect the engineering property of the rock; some of the

main units are classified into engineering geological subunits. These units and subunits are

dolerite (with subunits: highly weathered weak dolerite, moderately weathered medium strong

dolerite and fresh to slightly weathered strong dolerite), moderately weathered weak sandstone,

moderately to highly weathered weak limestone-marl- shale intercalation and slightly weathered

strong limestone.



85

5.2. Recommendations

Potentially expansive soils were identified in the town in many places. Hence, care should be

taken in constructing civil engineering structures on such type of soils. Moreover, expansive

soils in the town were found to cause failure of roads, buildings, drainage pipes and culverts. In

such areas proper investigation before design and close quality control during construction is

very crucial to minimize unnecessary cost and failure of infrastructures. Expansive soils are

difficult to use in the construction of highway, lightweight structures, construction of subsurface

drainage by concrete pipes, etc due to their swelling nature. Hence, removing part of the

expansive soil (moisture variation ranges 3 to 4 m) or lowering the foundation deeper than

normally used for stable soils is recommended. Further, detailed study on expansive soils (areal

distributions and mineralogical composition) and engineering properties, like swelling potential,

consolidation characteristics, rock mass properties, are also recommended.

6. ACKNOWLEDGMENT

The author acknowledges the Ethiopian Ministry of Science and Technology for providing the

partial grant for the research and the Department of Earth Sciences of Mekelle University for

providing logistic support. The technical support extended by CO-SAERT geotechnical

laboratory for analyzing soil samples is dully acknowledged. Drs. Nata Tadesse, Kifle

Weldearegay and K. Bheemalingeswara; and two anonymous reviewers deserve special thank for

their critical comments and constructive suggestions during the preparation of this manuscript.

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